Mass spectrometer



w. c. WILEY MASS SPECTROMETER Filed-Aug. 21, 1953 June 25, 1957 INVENTOR. W/LL/AM 6. MLEV ATTORNEV United States Patent 2,797,330 MASS SPECTROMETER William C. Wiley, Detroit, Mich., assignor to Bendix Aviation Corporation, Detroit, Mich., a corporation of Delaware Application August 21, 1953, Serial No. 375,634

7 Claims. (Cl. 25041.9)

This invention relates to mass spectrometers and more particularly to mass spectrometers for providing an enhanced resolution between ions of dilferent mass.

In some types of mass spectrometers, a plurality of ions are formed from the different molecules of gases and vapors in an unknown mixture. After a considerable number of ions have been formed, a force is imposed on the ions to accelerate the ions in a pulse from their place of retention mass receive a greater velocity than the ions of heavy mass and travel through a particular distance before the ions of heavy mass. By measuring the relative times at which the ions of different mass are detected, the masses of the different gases and vapors in the unknown mixture can be determined.

Ideally, optimum indications would be obtained if all the ions of a particular mass reached a detector at substantially the same instant of time. This does not occur in operation for a numberof reasons and often prevents a relatively sharp delineation from being made between ions of different masses. Part of the difiiculty results from a random motion of individual ions produced by thermal and other energies in the ions. Other troublesome causes may be imperfect construction and disposition of components in the spectrometer. Various features have been included in time-of-flight mass spectro meters to improve the resolution between ions of different masses. These innovations have been largely but not entirely successful.

This invention provides a mass spectrometer which produces an enhanced resolution between ions of ditferent mass over that obtained in the spectrometers now in use. The spectrometer includes an ion source, a detector disposed at a particular distance from the source and a bafile disposed between the ion source and the detector. A hole or slit is provided in the bafiie to provide a passage for the flow of ions from the source towards the detector. creased resolution between ions of different mass for reasons which are not clearly understood at present, Theories for such enhanced operation will be disclosed in detail hereinafter.

An object of this invention is to provide a mass spectrometer for determining the masses of different ions by measuring the times required for the ions of each mass to travel through a particular distance.

Another object is to provide a mass Spectrometer of the above character for obtaining anenhanced resolution between ions of difierent mass in comparison with the resolution obtained in spectrometers now in use;

A further object is to provide a mass spectrometer of the above character which includes a bafile disposed between an ion source and a detector to produce a con trolled flow of ions from the source to the detector.

Still another object is to provide a mass spectrometer of the above character for minimizing any undesirable eifects resulting from the imperfect construction and dis- The ions of relatively light.

The use of such a battle provides an in-.

position of components in an ion source in the spectrometer.

Other objects and advantages will be apparent from. a detailed description of the invention and from the appended drawings and claims.

The single figure is a somewhat schematic diagram, partly in block form and partly in perspective, illustrating one embodiment of the invention.

In one embodiment of the invention, a wedge-shaped filament 10 made from a suitable material such as tungsten is adapted to emit electrons when heated. An electrode 12 is disposed at a relatively short distance such as millimeter from the tip of the filament 10. The electrode 12 is provided with a vertical slot 14, the median position of which is at substantially the same horizontal level as the filament 10.

An electrode 16 is disposed in substantially parallel relationship to the electrode 12 and at a relatively short distance such as 2 millimeters from the electrode. The electrode 16 has a slot 18 corresponding substantially in shape and position to the slot 14. A collector 20 is disposed at a relatively great distance such as 4 centimeters from the electrode 16 and in substantially parallel relationship' to the electrode.

A backing plate 22 is positioned between the electrode 16 and the collector 20 and in substantially parallel relationship to. these members. The backing plate 22 is slightly to the rear of an imaginary line extending from the tip of the filament 10 through the slots 14 and 18 to the collector 20. An electrode 24 is substantially parallel to the backing plate 22 at a relatively short distance such as 2' millimeters from the plate. The electrode 24 is positioned slightly in front of the imaginary line disclosed above and is provided with a horizontal slot 26;

Top and bottom slats 28. made from a suitable insulating material extend between the backing plate 22 and the electrode 24 to form a compartment with these members. A horizontal slot 30 is provided in the bottom slat 28 at a position directly below the imaginary line disclosed above. A conduit 32 connects at one end withthe slot 30 and at the other end with a receptacle 34 adapted to hold the different molecules of gases and vapors in an unknown mixture. stantially parallel to the electrode 24 at a relatively small distance such as 2 millimeters in front of the electrode. The electrode 36 has a slot 38 corresponding substantially in shape and position to the slot 26 in the electrode 24. A detector such as a collector 40 is positioned at a relatively great distance such as 40 centimeters from the electrode 36. An indicator such as an oscilloscope 42- isconnected to the detector to indicate the relative times at which the ions of different mass are detected.

- A bafile'44- is disposed between the electrode 36 and the collector- 40 at a moderate distance such as /2 centimeter from the electrode. The baffle 44 is substantially parallel to the. electrode 3.6. It should be appreciated that the bafile 44- may be disposed. at other distances from the electrode 36 than that disclosed above. For example, the bafile may even be as much as 10 centimeters or more awayfrom, the electrode 36. The bafile has an aperture 46 which may be annular or rectangular or any other convenient shape. The dimension of the aperture 46' may varyover a considerable range. For example, a slit having a widthof one centimeter and a height of 35 centimeter has been successfully employed. Preferably, the aperture 46 should be somewhat smaller in size than the slots 38- and 26 in the electrodes. 36. and 24, respectively. Actually, more than one aperture may be successfully employed. For example, two apertures having widths of M4 centimeter and heights of Ms centimeter have. been disposed in a symmetrical An electrode 36 is subrelationship with respect to the slot 38 in the electrode 36. A direct voltage of positive polarity is applied to the electrode 12 through a resistance 50 from a suitable power supply 52. The collectors and 40 also have slightly positive voltages applied to them through suitable resistances 54 and 56, respectively, from the power supply. These voltages are applied to the collectors 20 and 40 so that the collectors will attract electrons secondarily emitted from them upon the impingement of charged particles. The filament 10, the backing plate 22 and the electrode 24 are connected to grounded resistances 58, 60.and 62, respectively, and the electrodes 16 and 36 and the bafile 44 are directly grounded. Negative pulses of voltage are applied to the filament 10 and the electrode 12 through coupling capacitances 64 and 66 from a suitable pulse forming circuit 68. Pulses of voltage are also applied to the backing plate 22 and the electrode 24 through suitable coupling capacitances 70' and 72, respectively, from the pulse forming circuit 68. As will be disclosed in detail hereinafter, the pulses of voltage applied to the backing plate 22 and the electrode 24 are of a positive polarity and occur at a later time than the pulses applied to the filament 10 and the electrode 12. A voltage pulse is also applied to the oscilloscope 42 to initiate the horizontal sweep of the oscilloscope beamat substantially the same instant as the .,application of pulses to the backing plate 22 and the electrode 24. Although the pulse forming circuit 68 is shown in block form, its construction and operation are known to persons skilled in the art. For example, model 903 of the Double Pulse Generator manufactured by the Berkeley Scientific. Instrument Company of Richmond, California, may be used to produce a plurality of pulses separated from one another by variable periods of time. The pulse forming circuit disclosed in co-pending application Serial No. 288,104, filed May 16, 1952, by Macon H. Miller and William C. Wiley, and now abandoned, can also be conveniently adapted for use.

In the steady state operation, the electrons emitted by the filament 10 are attracted to the electrode 12 because of the positive voltage on the electrode relative to the voltage on the filament. The electrons are decelerated in the region between the electrodes 12 and 16 since the electrode 16 is at substantially the same potential as the filament 10. This prevents electrons from traveling through the region between the backing plate 22 and the electrode 24 with a sufiicient energy to ionize molecules of gas and vapor introduced into the region from the receptacle 34. Upon the imposition of negative pulses of voltage on the filament 10 and the electrode 12, the voltage on the electrode 12 becomes negative with respect to the voltage on the electrode 16. This causes the electrons traveling through the slot 14 to be further accelerated in the region between the electrodes 12 and 16. Because of the acceleration imparted to the electrons in the regions between the electrodes 12 and 16, the electrons travel into the region between the backing plate 22 and the electrode 24 with a sufiicient energy to ionize molecules of the gases and vapors in the region. Most of the ions which are produced have a unitary positive charge.

The ions which are produced by the electron stream are retained in the electron stream because of their opposite charge relative to that of the stream. When a considerable number of ions have been produced for retention in the stream, the electron stream is interrupted by cutting off the voltage pulses on the filament 10 and the electrode 12. This makes the ions available for easy withdrawal upon the imposition of the voltage pulses on the backing plate 22 and the electrode 24.

The voltage pulses applied to the backing plate 22 and the electrode 24 have a magnitude to produce an electric field of moderate intensity between the plate 22 and the electrode 24 and an electric field of considerably increased intensity between the electrodes 24 and 36. For example, voltage pulses of +400 and +380 volts may be applied to the backing plate 22. and the electrode 24, respectively. By applying the particular voltages to the backing plate 22 and the electrode 24, compensation is provided for diiferences in the positioning and random motion of individual ions. Differences in thepositioning of individual ions result in part from the finite width of the electron stream. Differences in the random motion of individual ions result from thermal and other energy in the ions. The compensation provided by the creation of the particular electric fields is fully disclosed in co-pencling application Serial No. 249,318, filed October 2, 1951, by William C. Wiley, now Patent No. 2,685,035, issued July 27, l954.

The ions of light mass have a greater energy imparted to them by the electric fields between the backing plate 22 and the electrode 24 and between the electrodes 24 and 36 than the ions of heavy mass. After traveling through the slot 33 in the electrode 36, the ions travel to the collector 40 with substantially the same amount of energy as they had in moving through the slot 38. This results from the fact that substantially no electric field is produced in the region between the electrode 36 and the collector 40 since the electrode is at ground potential and the collector is at a potential only slightly above ground. During their travel through the region between the electrode 36 and the collector 40, the ions become materially separated on the basis of their mass. By measuring the relative times at which dilferent groups of ions reach the collector 40, the masses of the ions in each group can be determined.

I have found that an enhanced resolution can be obtained between the ions of ditferent mass by disposing the baffie 44 between the electrode 36 and the collector 40. The exact reason for such enhanced resolution is not apparent at present but several theories are plausible.

One theory results in part from the possibility that the backing plate 22 and the electrodes 24 and 36 are not always planar. It is also possible that these members may not be uniform in cross-section and may not be exactly parallel to one another. Even dirt on the walls of the backing plate 22 and the electrodes 24 and 36 may produce a phenomenon similar to non-uniformity in construction and disposition of these members.

The irregularities disclosed above may produce disturbances and discontinuities in the electrical fields so as to cause ions to travel in a path which is not exactly perpendicular to the backing plate 22 and the electrodes 24 and 36. Because of the aperture 46 in the baffle 44, a restriction is probably produced in the lateral area from which the detected ions originate. Since all of the ions originate from this restricted lateral area, it is possible that they travel through the same path such that all of the ions of a particular mass arrive at the collector at substantially the same instant of time.

The restriction in the area from which the detected ions originate may be desirable for another reason. This results from the possibility that the electric fields near edges of the slots 26 and 38 are dilferent from the electric fields at the centers of these slots. It is possible that the utilization of the bafile 44 and the aperture 46 may improve the resolution between ions of different mass by reducing such phenomena as edge etfects.

Under certain circumstances, the use of a baflie 44 may produce an increase in the number of ions which can be detected in each ion pulse before the occurrence of ion space charge saturation. For example, I have found that a 25 percent increase can be obtained in the number of ions in an unsaturated pulse when the aperture 46 has a lateral dimension of approximately 1 centimeter and a height of approximmately 0.2 centimeter.

Although this invention has been disclosed and illus- 76 trated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. The invention is, therefore, to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A mass spectrometer, including, a backing plate, a first electrode, means for providing a plurality of ions between the backing plate and the electrode, means for imposing an electric field on the ions in the region between the backing plate and the electrode to withdraw the ions from their place of provision and to produce a separation of the ions on the basis of their mass, an ion detector disposed at a particular distance past the electrode, means for producing a region having a particular electric field between the detector and the electrode, a baflie disposed in the region between the electrode and the detector, there being an aperture in the bafile to provide for the passage of ions to the detector, and means for indicating the relative times at which ions of different mass are detected.

2. A mass spectrometer, including, means for providing a plurality of ions and for retaining them in a particular region, means for imposing an accelerating force on the ions in a first region to produce a movement of the ions from their place of retention and a separation of the ions on the basis of their mass, an ion detector disposed at a particular distance past the end of the first region, means for providing between the first region and the detector a second region having a particular electrical field, a baffle disposed in the second region, there being an aperture in the bafiie to provide for the passage of ions towards the detector, and means for indicating the relative times at which the ions of different mass are detected.

3. A mass spectrometer, including, a backing plate, a first electrode, means for providing a plurality of ions and for retaining them in the region between the backing plate and the electrode, an electrical circuit for imposing voltages on the backing plate and the electrode to produce a withdrawal of the ions from their place of retention and a separation of the ions on the basis of their mass, 21 second electrode disposed at a par ticular distance from the first electrode, an ion detector positioned at a particular distance past the second elec trode and at a potential with respect to the electrode to produce substantially no electric field between them, a bathe disposed between the second electrode and the detector and having a potential to prevent distortion of the electrical field between the electrode and the detector, and means for indicating the relative times at which the ions of different mass are detected.

4. A mass spectrometer, including, a backing plate, an electrode, means for providing a plurality of ions between the backing plate and the electrode, means for imposing an electric field on the ions in the region between the backing plate and the electrode to withdraw the ions in a pulse and to produce a separation of the ions on the basis of their mass, an ion detector, means for providing a particular electrical field in a region between the electrode and the detector, a baffle disposed in the region between the electrode and the detector and having a potential to prevent distortion of the field, there being an aperture in the bafiie for providing a controlled movement of ions through the region, and means for indicating the relative times at which the ions of different mass are detected.

5. A mass spectrometer, including, a backing plate, a first electrode disposed at a particular distance from the backing plate and in parallel relationship to the plate, means for retaining a plurality of ions in the region between the backing plate and the first electrode, means for imposing a force of moderate magnitude on the ions in the region between the backing plate and the first electrode to withdraw the ions in a pulse from their place of retention and to produce a separation of the ions on the basis of their mass, 2. second electrode disposed at a particular distance from the first electrode and in substantially parallel relationship to the electrode, means for imposing a force of increased magnitude on the ions in the region between the first and second electrodes, an ion detector disposed at a particular distance from the second electrode, means for providing a substantially force-free region between the second electrode and the detector, 21 bafiie positioned between the second electrode and the detector and having a potential to prevent a disturbance of the force-free region, there being an aperture in the battle for controlling the movement of ions through the force-free region, and means for indicating the relative times at which the ions of dilferent mass are detected.

6. A mass spectrometer, including, means for providing a plurality of ions, means for imposing an electrical field on the ions in a first region until the movement of the ions past the region for the imposition of moderate veloci ties to the ions in the region and for the imposition of differences in velocities to the ions of each mass dependent upon the initial positioning of the ions, means for imposing an electrical field on the ions in a second region until the movement of the ions past the region for the imposition of substantially constant energies to the ions of each mass and energies greater than those imparted to the ions in the first region, an ion detector disposed at a particular distance past the end of the second region and having a potential to impose substantially no electrical field on the ions in a third region between the second region and the detector, a bafiie disposed in the third region and having a potential to prevent any disturbance in the field in the third region, there being an aperture in the bafile to provide for the movement of ions through the battle towards the detector, and means for indicating the relative times at which the ions of different mass are detected.

7. A mass spectrometer, including, means for providing a plurality of ions, an electrical circuit for imposing an electrical field upon the ions in a first region to produce a withdrawal of the ions from their place of provision and a separation of the ions on the basis of their mass, an ion detector positioned at a particular distance past the end of the first region, means for providing between the first region and the detector a second region having a particular electrical field, a baflle disposed in the second region at a potential in conformity with the electrical field in the region, there being at least one aperture in the baflie for providing for the passage of ions through the bafiie to the detector, and means for indicating the relative times at which ions of difierent mass are detected.

References Cited in the file of this patent UNITED STATES PATENTS Washburn Dec. 10, 1946 Backus Feb. 28, 1950 OTHER REFERENCES 

